Wheel drive system, in particular for an aircraft

ABSTRACT

A wheel drive system for ground movement of an aircraft includes a motor system and a clutch system. The motor system, which is borne by an unsuspended part of a landing gear strut of the aircraft, includes an electric motor and a reduction gearing system. The clutch system, which connects an output shaft of the electric motor to a wheel of the aircraft via the reduction gearing system, includes a dog-clutch and a translational movement system. The dog-clutch includes a drive part, which is secured to the motor system, and a receiving part, which secures to a tire of the wheel. The translational movement system moves the drive part along an axis of an axle crossbeam of the landing gear strut into an engaged position, in which the drive part collaborates with the receiving part, and a disengaged position, in which the drive part is separated from the receiving part.

FIELD OF THE INVENTION

The present invention relates to the field of aircraft and notably thesystems for driving a wheel when the aircraft is moving around on theground.

RELATED ART

A distinction is generally made between a number of phases in which anaircraft is moving around on the ground prior to the phase of take-offor after the phase of landing: a phase of moving around at very lowspeed, for example less than 5 km/h, commonly referred to as the“manoeuvring phase” aimed at moving the aircraft from a parking/storagelocation to a loading zone, and a phase of moving around in forward gearat low speed, for example of the order of 20 km/h, more commonly anduniversally known by its English name of “taxiing”. The manoeuvringphase may include movement in reverse gear, for example in order to exita building such as a storage hangar and/or moving around in forward gearin order to reach a loading zone. The movements during the manoeuvringphase are performed at very low speed, generally with assistance fromindividuals on the ground outside the aircraft.

The phase of manoeuvring in reverse gear and/or in forward gear at verylow speed is generally performed using a motor vehicle able to steer theaircraft, such as, for example, a dedicated tractor more commonlyreferred to by its English name of “tug”.

One solution is to associate an electric motor with the aircraft wheel.However, permanent connection between the wheel and the motor means thatthe electric motor has to rapidly reach a high rotational speed during ahigh-speed landing, for example at 220 km/h, and that entails the use ofan electric motor capable of withstanding such a speed. In addition,because of the permanent connection between the wheel and the motor, thewheel may become locked if the electric motor or the transmission meansbecome blocked.

Furthermore, such systems do not provide the ability to withstanddeformations of the suspension strut and of the wheel, notably duringthe various phases of manoeuvring and/or taxiing on the ground, such asthe aircraft executing turns and/or braking.

Reference may be made to document FR 2 975 340 which describes a devicefor the rotational coupling of an output gearwheel of a reductiongearset to a wheel of an aircraft comprising a plurality of link rodsforming ball-jointed connections with the rim of the wheel and theoutput gearwheel respectively.

However, such a coupling device does not allow the drive system to adopta disengaged position in which the motor unit is uncoupled from thewheel, which means that during phases of high-speed running, such as,for example, when the aircraft is landing, the electric motor needs tobe capable of withstanding such a speed.

Reference may to made to document EP 2 527 249 which describes adisengageable interface mechanism for the interface between a wheeldrive system of an aeroplane landing gear and a wheel comprising ageared motor unit and a clutch system within the geared motor unit. Theinterface mechanism comprises a rotary part rotationally driven by thegeared motor unit and a fixed part secured to the rim and permanentlyengaging with the rotary part. The coupling between the geared motorunit and the wheel is achieved by the intermeshing of one of theintermediate pinions of the geared motor unit and a gearwheel by virtueof systems that block the rotation of the intermediate pinions borne bya pendular element articulated about the axis of rotation of the drivepinion and adjustable end stops that adjust the angular travel of thependular element.

However, such a pivoting system is particularly complex and is likewiseunable to withstand deformations between the suspension strut and thewheel. In addition, such a system entails permanent collaborationbetween the rotary part of the interface mechanism and the rim of thewheel.

Reference may be made to document WO 2011/073590 which proposes anautonomous drive system for an aircraft wheel comprising a motor unitconnected in pivoting fashion to a non-suspended part of a suspensionstrut, a drive member, such as a ring gear, secured to the rim of thewheel and a clutch device borne by the suspension strut and allowing theoutput shaft of the motor unit to be connected to the drive member. Theclutch system allows the motor unit and the wheel to be engaged ordisengaged. The motor unit is able to move with respect to the wheel viathe drive member into an engaged position during movement on the groundat low speed and disengaged during landing or take-off. In order tocompensate for deformations of the landing gear, a constant-velocityjoint is positioned between the output pinion of the motor unit and theoutput shaft of the motor unit supporting the output pinion.

However, such a system is complex to produce and entails the use of aplurality of link rods rotationally mounted on the non-suspended part ofthe suspension strut and on the motor unit.

BRIEF DESCRIPTION OF EMBODIMENTS OF THE INVENTION

It is therefore an object of the present invention to overcome thesedisadvantages.

More specifically, the present invention seeks to provide a wheel drivesystem incorporated into the landing gear and allowing the aircraft tobe moved around during the manoeuvring phase and during the taxiingphase, while at the same time affording protection to the drive system,for example during landing, take-off and aircraft movements at highspeed.

Another objective of the present invention is to allow the drive systemto withstand the deformations of the suspension strut and of the wheel,notably during the various phases of manoeuvring on the ground and oftaxiing, such as when the aircraft is executing turns and/or braking.

One subject of the invention is a wheel drive system, for the groundcirculation of an aircraft, comprising a motor unit borne by anunsuspended part of a landing gear strut of the aircraft and comprisingan electric motor and reduction means. The wheel drive system furthercomprises a clutch device connecting the output shaft of the electricmotor to the wheel via the reduction means.

The clutch device comprises a dog-clutch mechanism comprising a drivepart secured to the motor unit and a receiving part secured to the tireof the wheel, and a system for the translational movement, along theaxis of the axle crossbeam of the landing gear strut, of the drive partinto an engaged position in which the drive part collaborates with thereceiving part and a disengaged position in which the drive part isseparated from the receiving part.

Such a drive system is secured to the landing gear and more specificallyto the axle crossbeam that bears the wheel. It is therefore carrieddirectly on board the aircraft.

In one embodiment, the receiving part comprises a plurality ofshock-absorbing pads projecting axially towards the drive part. Theshock-absorbing pads are secured to one of the lateral sidewalls of thetire and spaced apart in such a way as to leave a space between twoadjacent shock-absorbing pads.

The shock-absorbing pads are, for example made of a rubbery material andovermoulded directly onto the lateral sidewall of the tire.

The shock-absorbing pads may be made from a material that differs fromthe material used for the tire, notably in the region of the lateralsidewalls. For example, the shock-absorbing pads are made from amaterial that has a hardness of 35 Shore A.

In one embodiment, the drive part comprises an annular part or annulusfixed to the motor unit and a plurality of coupling fingers or dogsfixed uniformly around the circumference of the annulus and intended toengage, when the wheel drive system is in the engaged position, in arespective square-edged slot between two adjacent shock-absorbing padsof the receiving part.

In one embodiment, the translational movement system comprises aconnecting piece secured to the reduction gearbox of the motor unit andmounted on the axle crossbeam of the landing gear strut.

The connecting piece may be mounted on two guide pins secured to thelanding gear strut, notably the axle crossbeam.

By way of nonlimiting example, one of the two guide pins may becylindrical and the second guide pin parallel to the first may have ashape having two planar surfaces oriented judiciously so as to avoid astatically indeterminate construction and withstand potentialdimensional, notably thermal, variations better.

In one embodiment, the system for moving the clutch device comprises anelectric actuator fixed to the landing gear strut, notably on the axlecrossbeam, and comprising an actuator rod mounted via a slidingconnection in the actuator and one end of which is fixed to theconnecting piece.

The clutch device may comprise means for unblocking and blockingtranslational movement, allowing the wheel drive system to be kept in adisengaged position. The translational unblocking and blocking means maybe electrically operable. The translational blocking may come intoeffect through a lack of current and via the use of a spring.

The means for unblocking and blocking translational movement for examplecomprise an actuating cylinder, secured to the connecting piece,comprising a piston, that is actuated by an electromagnet, a first endof which is connected to the actuating cylinder body by a compressionspring and a second end of which is intended to engage in acorresponding housing on one of the guide pins when the wheel drivesystem is in the non-engaged position of rest.

In one embodiment, the reduction means comprise a drive pinion connectedto the output shaft of the electric motor, a planetary reductiongearset, for example with at least two reduction stages, rotationallydriven by the drive pinion and driving a large-diameter, reductiongearset output bevel annulus gear via a reduction gearset output bevelpinion. The reduction gearset output bevel pinion thus meshes with thereduction gearset output bevel annulus gear, allowing an angulartransmission, for example through 100°.

The drive part of the clutch device may be secured, permanently, to thelast reduction stage of the reduction means, such as, for example, thereduction gearset output bevel annulus gear. Thus, the clutch device isexternal to the motor unit.

The supply of power to the electric motor of the drive system, and tothe system for moving and the means that unblock and block thetranslational movement of the drive part in a non-engaged position ofrest may come from a power source present on the aircraft, such as anon-board battery, or from an auxiliary generator onboard the aircraft.

According to a second aspect, the invention relates to an aircraftcomprising at least one wheel provided with a wheel drive system asdescribed hereinabove.

In one embodiment, the aircraft comprises at least one landing gearprovided with two wheels at least one of which wheels is provided with awheel drive system as described hereinabove.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects, features and advantages of the invention will becomeapparent from reading the following description, given solely by way ofnonlimiting example and made with reference to the attached drawings inwhich:

FIG. 1 depicts a perspective view of part of a landing gear of anaircraft equipped with two wheels and with a wheel drive systemaccording to the invention;

FIGS. 2 and 3 depict a rear view of part of a landing gear according toFIG. 1, illustrating the wheel drive system in a position in which it isdisengaged from the wheel and in a position in which it is engaged withthe wheel, respectively;

FIG. 4 depicts a perspective view of a driven wheel;

FIG. 5 depicts a view in section V-V according to FIG. 2;

FIG. 6 depicts detail of the clutch device of the wheel drive systemaccording to the invention;

FIGS. 7 and 8 depict views in section VII-VII and VIII-VIII according toFIG. 5 of the clutch device in the disengaged and engaged position;

FIG. 9 depicts a view in section IX-IX according to FIG. 5; and

FIG. 10 depicts a view in section showing in detail how the drive partis fixed to the motor unit.

In what follows of the description, the terms “longitudinal”,“transverse”, “vertical”, “front”, “rear”, “left” and “right” are to beunderstood in relation to the usual orthogonal frame of reference ofaircraft, depicted in FIG. 1 and comprising:

-   -   a longitudinal axis X, which is horizontal and directed from the        front towards the rear of the aircraft;    -   a transverse axis Y, which is horizontal and perpendicular to        the axis X and directed from right to left of the aircraft        moving forward;    -   an axis Z, orthogonal to the axes X and Y and vertical, directed        from the bottom upward.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

As illustrated in FIG. 1, part of a landing gear of an aircraft, whichmay for example be a fleet airliner, referenced 10 overall, comprisestwo wheels 12, just one of which has been depicted in the drawings, alanding gear strut 14, supporting the wheels 12, a drive system 16 fordriving one of the two wheels 12 so that the aircraft can move around onthe ground, and a braking system 18 associated with each of the wheels,visible in detail in FIG. 10. By way of nonlimiting example, the landinggear may be one of the main landing gears of an aircraft comprising twomain landing gears.

The landing gear strut 14 comprises a suspended part 14 a connected tothe structure of the aircraft and enabling absorption of the energy ofthe impact of landing and movements around on the ground while at thesame time affording the passengers onboard the aircraft the maximumlevel of comfort, and a non-suspended part 14 b connected to thesuspended part and comprising an axle crossbeam 20 extending along thehorizontal axis Y of the aircraft.

The two wheels 12 each comprise a tire 12 a and a rim 12 b inserted inthe tire and mounted to rotate on the axle crossbeam 20 that acts as asteering knuckle, via rolling bearings (which have not been depicted).

The wheel drive system 16 is borne by the non-suspended part 14 b of thelanding gear strut 14 and comprises a motor unit 22 and a clutch device24 which is intended to couple and uncouple the output shaft of themotor with respect to one of the wheels 12.

The motor unit 22 comprises an electric motor 26, such as, for example,a permanent-magnet three-phase synchronous motor, and reduction meanswhich will be described in detail with reference to FIG. 10. Thereduction means are housed in a reduction gearbox 28 secured to aconnecting piece 30 used for connection to the axle crossbeam 20 of thelanding gear strut 14.

The clutch device 24 comprises a dog-clutch mechanism comprising a drivepart 32 secured to the motor unit 22 and a receiving part 34 secured tothe wheel 12, notably to the tire 12 a.

The clutch device 24 further comprises a system for the translationalmovement, along the axis of the axle crossbeam 20 of the landing gearstrut 14, of the drive part 32 into a disengaged or declutched position,visible in FIG. 2, in which the drive part 32 is separated from thereceiving part 34 and an engaged or clutch-engaged position, visible inFIG. 3, in which the drive part 32 collaborates with the receiving part34.

As illustrated in detail in FIG. 4, the receiving part 34 comprises aplurality of shock-absorbing pads 40, of parallelepipedal overall shape,arranged uniformly on one of the lateral sidewalls 12 c of the tire,leaving a space or square-edged slot 38 between two adjacentshock-absorbing pads 40.

The shock-absorbing pads take the form of projections extending axiallyfrom one of the lateral sidewalls 12 c of the tire 12 a towards thedrive part 32.

As illustrated, the shock-absorbing pads 40 are overmoulded directly onthe lateral sidewall 12 c. The shock-absorbing pads may be made from amaterial that differs from the material used for the tire, notably interms of hardness, so as to transmit the rotation torque to the wheel bycontact with the drive part 32.

Specifically, the lateral sidewalls of a tire are generally made from arubber that is soft so as to tolerate the deformations on eachrevolution of the wheel while at the same time being resistant toimpact. The shock-absorbing pads 40 need to have a greater hardness thanthe rubber used for the lateral sidewalls. By way of nonlimitingexample, the shock-absorbing pads may be made from a rubbery materialhaving a hardness of 35 Shore A.

As illustrated in detail in FIG. 10, the drive part 32 comprises anannulus 42 fixed to the motor unit 22 and a plurality of couplingfingers 44 or dogs fixed uniformly to the circumference of the annulus42, by fixing means 46, for example screw-fasteners. Each couplingfinger 44 comprises a fixing part (unreferenced) for fixing to theannulus 42 and a part (unreferenced) projecting axially along thehorizontal axis Y towards the receiving part 34 and projecting radiallytowards the outside of the drive part 32. The coupling fingers 44 areintended to engage, when the system that drives the wheel 12 is in theengaged position, in a respective square-edged slot 38 between twoadjacent shock-absorbing pads 40 of the receiving part 34. The couplingfingers are made of a metallic material, such as steel for example.

The rotation of the electric motor 26 is transmitted by contact betweenthe coupling fingers 44 and the shock-absorbing pads 40. Specifically,as the electric motor 26 turns, each coupling finger 44 presses againstone side of a shock-absorbing pad which drives the wheel 12 in rotation.

The compression of the shock-absorbing pads 40 and the sliding of thefingers 44 make it possible to absorb deformations between the wheel 12and the landing gear strut 14. Specifically, the deformation of theshock-absorbing pads 40 tolerates the deformations of the landing gear10 and of the wheel 12.

It is necessary to provide square-edged slots 38 that are large enoughin the circumferential direction that they can accept the couplingfingers 44 even when the deformations of the landing gear 10 aresignificant.

The connecting piece 30 fixed to the axle crossbeam 20 of the landinggear strut 14, which is visible in detail in FIG. 5, in a nonlimitingmanner has a partially circular particular shape in order notably tominimize its weight.

As illustrated in detail in FIG. 6, the system for movement of theclutch device 24 further comprises a remotely actuated electric actuator48 comprising an actuator rod 48 a mounted in a sliding connection inthe actuator 48 and one end of which is fixed to the connecting piece30. The base of the actuator 48 is fixed to two fixing lugs 20 a of theaxle crossbeam 20. The electric actuator 48 is of known type and willnot be described further.

The system for movement of the clutch device 24 comprises twotranslational-guidance guide pins 50, parallel to one another, each end50 a, 50 b of which is fixed to a respective fixing lug 20 a of the axlecrossbeam 20, the axle crossbeam 20 comprising four fixing lugs 20 a.

As illustrated in detail in FIGS. 7 and 8, the connecting piece 30comprises two housings 30 a each intended to accept a guide pin 50between two bearings (unreferenced). Thus, when the actuator rod 48 amoves in the direction of the arrow F, the connecting piece 30, which isprevented from rotating by the two guide pins 50, is set intranslational motion along the two guide pins 50 and thus forms asliding connection with the axle crossbeam 20.

Because the drive part 32 is secured to the connecting piece 30 by themotor unit 22, the movement system allows the drive part 32 atranslational movement, along the axis of the axle crossbeam 20 of thelanding gear strut 14, with respect to the receiving part 34.

The clutch device 24 comprises translational unblocking and blockingmeans 52 that allows the system for driving the wheel 12 to be kept in adisengaged position. The translational unblocking and blocking means 52comprise an actuating cylinder 52 a secured to the connecting piece 30and actuated by an electromagnet (unreferenced). One end of the piston52 b of the actuating cylinder is connected to the actuating cylinderbody by a compression spring 52 c. When the aircraft is not in thetaxiing phase, or when there is no longer any electromagnet supplycurrent, the end of the piston 52 b is pressed against the exteriorsurface of the upper guide pin 50. If the system for driving the wheel12 is in a disengaged position, the end of the piston 52 b becomesinserted in a corresponding hole 50 c made in the upper guide pin 50.Thus, the connecting piece 30 and, therefore, the drive part 32, cannotmove towards the receiving part 34. The mechanism internal to theactuating cylinder 52 a will not be described further. As illustrated, afirst position detector 54 a fixed to the actuating cylinder 52 a makesit possible to ensure that the end of the piston 52 b is disengaged fromthe upper guide pin 50, and a second position detector 54 b fixed to thelanding gear 14, for example to a fixing lug 20 a, makes it possible toconfirm that the end of the piston 52 b is in the disengaged position.

The actuator 48 and the translational blocking and unblocking means 52may be supplied with electrical power by a power source internal to theaircraft, such as, for example, the electricity produced, for example bythe aircraft auxiliary electric generator system or alternatively abattery carried onboard or any other source of power of the aircraft.

During the taxiing phase, the drive part 32 is moved by means of theactuator 48 of the movement system towards the receiving part 34 so asto allow the dog-clutch mechanism to drive the wheel 12 in rotation. Thedrive to the wheel 12 is therefore completely uncoupled during thephases of take-off, landing and flight.

As illustrated in detail in FIG. 9, the reduction means comprise a drivepinion 56 connected to the output shaft of the motor 26 a, meshing witha two-stage planetary reduction gearset 60 rotationally driving a bevelannulus gear by means of a reduction gearset output bevel pinion.

As an alternative, a lower or higher number of reduction stages could beprovided between the driving pinion and the output annulus gear of thereduction gearset.

As illustrated, the planetary reduction gearset 60 with two reductionstages comprises a plurality of first planet pinions 61 mounted with thefreedom to rotate on spindles (not depicted) the ends of which aresecured to opposite grooves of a first planet carrier 62. The firstplanet pinions 61, of which there are, for example, three, mesh with thedrive pinion 56 and a first internal toothset 28 a formed in thereduction gearbox 28, which is prevented from rotating. As analternative, an annulus gear could be provided, fixed to the inside ofthe reduction gearbox and prevented from rotating. Thus, rotation of thedrive pinion 56 drives the first planet carrier 62 in rotation.

The planetary reduction gearset 60 further comprises a plurality ofsecond planet pinions 63 with the freedom to rotate on spindles (notdepicted) the ends of which are secured to opposite grooves of a secondplanet carrier 64 mounted with the ability to rotate with respect to thefirst planet carrier 62 via rolling bearings (unreferenced). The secondplanet pinions 63, of which there are, for example, three, mesh with atoothset 62 a of the first planet carrier 62 and a second internaltoothset 28 b formed in the reduction gearbox 28, which is preventedfrom rotating. As an alternative, an annulus gear could be provided,fixed to the inside of the reduction gearbox. Thus, rotation of thefirst planet carrier 62 drives the rotation of the second planet carrier64. The planet pinions 61, 63 comprise, on their periphery, toothsets(unreferenced) which may be straight-cut or helical.

The second planet carrier 64 comprises an external spline 64 a meshingwith an internal spline 66 a of an output shaft 66 of the reductiongearset mounted to rotate in the reduction gearbox 28 and bearing areduction gearset output bevel pinion 67. The reduction gearset outputbevel pinion 67 meshes with a reduction gearset output bevel annulusgear 68, providing an angular transmission. The bevel annulus gear 68has a large diameter and is mounted in an annulus gear case 70 via alarge-diameter rolling bearing (unreferenced). As illustrated, theconnecting piece 30 is fixed to the annulus gear case 70.

As illustrated, the reduction gearset output pinion and annulus gear 67,68 use bevel toothsets. As an alternative, any similar type of toothsetcould be employed.

As illustrated, the reduction system is aligned with the motor so thatthe output shaft of the motor 26 a, the two planet carriers 62, 64 andthe output shaft of the reduction gearset 66 are coaxial. As analternative, provision could be made for the reduction system not to bealigned with the motor.

The annulus 42 of the drive part 32 of the clutch device 24 ispermanently secured to the reduction gearset output bevel annulus gear68 so that the drive part 32 is rotationally driven by the electricmotor 26 via the reduction means. Thus, the clutch device 24 is externalto the motor unit 22 because it is connected to the final reductionstage of the reduction means of the motor unit.

One example of how the annulus 42 of the drive part 32 of the clutchdevice 24 is fixed to the bevel annulus gear 68 is illustrated in FIG.10. However, it will be noted that any other fixing system that allowsthe drive part to be attached to the bevel annulus gear of the motorunit may be provided.

As illustrated and entirely nonlimitingly, the annulus 42 comprises aplurality of fixing lugs 42 a extending radially outwards and arrangedin such a way as to leave, between two adjacent lugs, a space foraccommodating a pad 68 b secured to the bevel annulus gear 68. Axialretention of the pad 68 b in the receiving lugs 42 a is achieved by anaxial retention means 42 b fixed to the bevel annulus gear 68.

As illustrated in FIG. 9, the braking system 18 associated with each ofthe wheels 12 is secured to the axle crossbeam 20 and mounted with theability to rotate via rolling bearings (not depicted) with respect tothe rim 12 b of the associated wheel 12. It will be noted that only thevolume allotted to the braking system 18 has been depicted in thefigures, which means that the braking system itself is not detailed andwill not be described further.

The method for the dog-coupling or engaging of the wheel 12 drive systemduring the taxiing phase is as follows:

Initially, the drive part 32 of the clutch system 24 is in the restposition, which means to say the disengaged position. This position ismaintained by the translational unblocking and blocking means 52.

The coupling fingers 44 of the drive part 32 are set in rotation at aspeed that differs slightly from the rotational speed of the wheel 12,which speed may be supplied, for example, by a speed sensor (notdepicted). For this purpose, an electronically controlled speed-matchingsystem (not depicted) associated with the clutch device may be provided,allowing control over the rotational speed of the electric motor as afunction of the measured rotational speed of the wheel.

The torque of the electric motor 26 is then set to a minimum, then thetranslational unblocking and blocking means 52 are actuated in order tounlock the drive part 32 from the rest position, which means to say inorder to retract the end of the piston 52 b out of the hole 50 c made inthe upper guide pin 50.

The drive part 32 is moved, via the connecting piece 30, in thedirection of the arrow F, towards the receiving part 34 secured to thetire 12 a using the actuator 48.

As soon as the coupling fingers 44 of the drive part 32 and theshock-absorbing pads 40 of the receiving part 34 come into contact, theforce of the actuator 48 becomes increased, which force can becontrolled through control of the actuator motor.

Because of the difference between the rotational speed of the wheel andthe rotational speed of the drive part, the coupling fingersprogressively engage in the square-edged slots of the receiving part.The engaged/coupled position can be detected, for example, by theencoder of the motor of the actuator.

Because of the dog-clutch mechanism, the wheel drive system accommodatesdeformations between the wheel and the landing gear strut by means ofthe elastically compressible pads. Engagement/disengagement of the drivepart with respect to the receiving part, and absorption of deformationoccur at a single point, at the level of the tire.

During phases of flight other than the phases of taxiing and manoeuvringon the ground, the drive and receiving parts of the clutch device arecompletely separated from one another.

In addition, the drive system can easily be adapted to suit the existinglanding gears, with very little modification.

The invention applies advantageously to an aircraft comprising twowheels, just one of which is motorized by the drive system describedhereinabove. Provision could be made for each wheel of the landing gearto be provided with a wheel drive system as described hereinabove.Provision could also be made for the aircraft landing gear to comprise,in addition to the pair of motorized wheels, two twinned wheels. Thewheel drive system described may also be applied to an aircraftcomprising one wheel equipped with the wheel drive system. Specifically,it is possible to motorize only a forward landing gear comprising asingle wheel or a pair of wheels, generally referred to as “nose gear”.Provision could be made for the said motorized wheel to be steered,namely associated with an orientation system, such as, for example, anelectric actuating cylinder.

In the case of an aircraft comprising at least one non-orientable mainlanding gear and one orientable landing gear, the motorizing of thewheels is preferably performed on at least one of the wheels of the mainlanding gear. The orientable landing gear may, for example, be a tailwheel the orientation of which may be locked, for example, during thetaxiing phase.

The invention claimed is:
 1. A wheel drive system for ground movement of an aircraft, the wheel drive system comprising: a motor system supported by an unsuspended part of a landing gear strut of the aircraft, the motor system including: (a) an electric motor, and (b) a reduction gearing system; and a clutch system, which connects an output shaft of the electric motor to a wheel of the aircraft via the reduction gearing system, the clutch system including: (a) a dog-clutch including (i) a drive part, which is secured to the motor system, and (ii) a receiving part, which is secured to a tire of the wheel, and (b) a translational movement system, which translationally moves the drive part along an axis of an axle crossbeam of the landing gear strut into (i) an engaged position, in which the drive part collaborates with the receiving part, and (ii) a disengaged position, in which the drive part is separated from the receiving part, wherein the receiving part includes a plurality of shock-absorbing pads overmolded directly on a lateral sidewall of the tire and extending axially from the lateral sidewall towards the drive part, the plurality of shock-absorbing pads being circumferentially spaced apart so as to leave a square-edged slot between each adjacent pair of the shock-absorbing pads.
 2. The wheel drive system according to claim 1, wherein the shock-absorbing pads are made of a rubbery material.
 3. The wheel drive system according to claim 2, wherein the shock-absorbing pads are made from a material that differs from a material used to form the tire.
 4. The wheel drive system according to claim 1, wherein the drive part includes: an annulus fixed to the motor system, and a plurality of coupling fingers fixed uniformly around a circumference of the annulus such that, in the engaged position, the coupling fingers engage respectively with the square-edged slots.
 5. The wheel drive system according to claim 1, wherein the translational movement system includes a connector secured to the reduction gear system of the motor system, the connector being slidingly mounted on two guide pins secured to the axle crossbeam of the unsuspended part of the landing gear strut.
 6. The wheel drive system according to claim 5, wherein the translational movement system further includes: an electric actuator fixed to the unsuspended part of the landing gear strut, and an actuator rod slidingly connected to the electric actuator, with one end of the actuator rod being fixed to the connector, the translation movement of the actuator rod being translated into a translation movement of the connector along the guide pins.
 7. The wheel drive system according to claim 1, wherein the clutch system includes a blocker, which blocks the translational movement system to cause the drive part to be kept in the disengaged position.
 8. The wheel drive system according to claim 7, wherein the translational movement system includes a connector secured to the reduction gear system of the motor system, the connector being slidingly mounted on two guide pins secured to the axle crossbeam of the unsuspended part of the landing gear strut, and wherein the blocker includes an actuating cylinder secured to the connector, the actuating cylinder including an electromagnetically actuatable piston comprising (a) a first end connected to a body of the actuating cylinder by a compression spring and (b) a second end that engages in a corresponding housing on one of the guide pins when in the disengaged position.
 9. The wheel drive system according to claim 1, wherein the reduction gearing system includes: a drive pinion connected to the output shaft of the electric motor, and a planetary reduction gearset rotationally driven by the drive pinion and arranged to drive a reduction gearset output bevel annulus gear via a reduction gearset output bevel pinion.
 10. The wheel drive system according to claim 9, wherein the drive part of the dog-clutch is fixed to the reduction gearset output bevel annulus gear.
 11. The wheel drive system according to claim 9, wherein the planetary reduction gearset includes at least two reduction stages.
 12. The wheel drive system according to claim 1, wherein the wheel drive system is incorporated in the aircraft.
 13. An aircraft comprising: (a) a wheel; (b) a landing gear strut; and (c) a wheel drive system for ground movement of the aircraft, the wheel drive system including: (1) a motor system supported by an unsuspended part of the landing gear strut, the motor system including: (a) an electric motor, and (b) a reduction gearing system, and (2) a clutch system, which connects an output shaft of the electric motor to the wheel via the reduction gearing system, the clutch system including: (a) a dog-clutch including (i) a drive part, which is secured to the motor system, and (ii) a receiving part, which secures is secured to a tire of the wheel, and (b) a translational movement system, which translationally moves the drive part along an axis of an axle crossbeam of the landing gear strut into (i) an engaged position, in which the drive part collaborates with the receiving part, and (ii) a disengaged position, in which the drive part is separated from the receiving part, wherein the receiving part includes a plurality of shock-absorbing pads overmolded directly on a lateral sidewall of the tire and extending axially towards from the lateral sidewall towards the drive part, the plurality of shock-absorbing pads being circumferentially spaced apart so as to leave a square-edged slot between each adjacent pair of the shock-absorbing pads.
 14. The aircraft according to claim 13, further comprising a landing gear system that includes the landing gear strut and a pair of wheels, the pair of wheels including the wheel and a second wheel, wherein (1) the wheel and the second wheel share the wheel drive system or (2) the second wheel is separately equipped with its own wheel drive system. 